Polycarbonate resins have been widely used as engineering plastics due to their excellent impact resistance, self-extinguishing property, dimensional stability, and high thermal resistance as compared to other resins. However, the polycarbonate resin has a very low rigidity (flexural strength) due to its amorphous structure so that it has been limited in application to thin-film injection products.
Such a problem can be overcome by mixing filler materials such as glass fiber with the polycarbonate resin, and introduction of the reinforcing fiber into a polymerized resin product can improve tensile strength, creep and fatigue resistance, and thermal-expansion resistance, as well as rigidity. However, it also causes a serious deterioration in impact resistance of a polycarbonate resin composition.
One solution to this problem is to employ a milled glass fiber in a large or small amount. However, this approach can provide a slight improvement in impact strength with a predetermined content of the glass fiber, but involves a decrease in effect of improving the rigidity.
Another approach is reinforcement of the resin with filament fillers instead of introduction of staple fillers into the resin. However, effective filling of the filament fillers is very difficult due to a high viscosity of the polycarbonate composition which is an amorphous resin.
As a result of extensive studies to solve the above problems, the inventors of the present invention found that a resin composition exhibiting excellent impact resistance while maintaining high rigidity could be prepared by adding a filament filler to a high-fluidity polycarbonate resin having a weight-average molecular weight of 25,000 g/mol or less to form a master-batch, and blending the master-batch with a high-molecular weight polycarbonate resin having a weight-average molecular weight of more than 25,000 g/mol or with a master-batch of a rubber-modified styrene graft copolymer resin and a styrene copolymer resin.